Voltage controlled oscillators have been used in the communications industry typically for portable radios, portable telephones and pagers. However, the bandwidth of prior art voltage controlled oscillators may not be sufficient for certain applications. For example, in U.S. Pat. No. 4,450,416 a wideband voltage controlled oscillator is described which has a resonant circuit comprising varactors, an inductor and capacitor. A control voltage is applied to cathodes of the varactors to alter their capacitance. Accordingly, the frequency of the voltage controlled oscillator can be controlled by varying the control voltage. One problem with this oscillator is that it requires a relatively small value of capacitance in the resonant circuit which is therefore a limitation on bandwidth.
Disclosed in U.S. Pat. No. 5,144,264 is an open loop gain compensated wideband Colpitts voltage controlled oscillator having a direct current drain that is flatter than other prior art Colpitts voltage controlled oscillators. However, the bandwidth of this wideband voltage controlled oscillator may not be sufficient for all applications.
Hartley wideband voltage controlled oscillators are often used to obtain a wider bandwidth coverage than is achievable by Colpitts voltage controlled oscillators. One disadvantage with conventional Hartley wideband voltage controlled oscillators is that the direct current drain decreases with frequency. This decrease in direct current drain can affect phase noise performance which is undesirable for certain applications.
Unfortunately, one significant problem of prior art radio frequency wideband voltage controlled oscillators is that they do not provide for a relatively constant SideBand Noise Ratio (SBNR) across their bandwidth and therefore this may affect selectivity of adjacent channels.